The initial step in the action of luteinizing hormone (LH) in the stimulation of steroidogenesis in luteal tissue is its binding to specific receptors at the plasma membrane. Both the total number of receptors and the number occupied by endogenous LH are highly correlated with the functional state of the corpus luteum. Therefore, factors regulating the loss and renewal of LH receptors are of primary importance in the regulation of progesterone secretion. The first objective of this research is to investigate the pathways of loss of LH receptors. Biologically active 125I-hCG and cationic ferritin will be used as markers to elucidate the subcellular compartments involved in internalization, degradation and recycling of hormone-receptor complexes and plasma membrane, respectively. Electron microscopic (EM) autoradiograhic techniques will be used to follow hormone-receptor complexes. The design of the proposed experiments should make it possible to determine if loss of LH receptor is specific or due to generalized turnover of plasma membrane. The second objective is to ascertain the pathway(s) for renewal of receptors. To determine the subcellular compartments involved in receptor renewal, procedures to make intracellular receptor sites accessible for binding to 125I-hCG (i.e., mild fixation and brief treatment with saponin) will be used in conjunction with EM autoradiography. In addition, experiments have been designed to determine if the limiting membrane of secretory granules (which are incorporated into the plasma membrane as a consequence of membrane fusion and exocytosis) contain receptors for LH. Therefore, results from the experiments included in this application should provide new information essential to a complete understanding of how luteal secretion of progesterone is regulated, the implications of which are important to the regulation of human fertility and increased reproductive efficiency in domestic animals.